1. Field of the Invention
The present invention relates to a correlator (matched filter) for examining the correlation between an input impulse train (string) and a time-series signal, and particularly relates to a correlator for UWB (Ultra Wide Band) communication and a receiver including the correlator.
2. Description of Related Art
Recent explosive growth of various information-communication devices typified by mobile phones and PDAs has accelerated the shortage of available frequency bands. In the meantime, today's highly-networked information society boosts demand for multimedia mobile communications utilizing time-varying (animation) images and great amounts of data.
In this background, what has been researched is impulse radio (Ultra Wide Band Radio, UWB Radio) technology which opens the way for realizing communication or instrumentation (radar) without causing interferences with conventional radio communications, by means of impulses with minimum power spectral density, which can broadly spread the spectrum (from very low frequency to GHz range) (cf. Puneet Newaskar, Raul Blazquez, and Anantha Chandrakasan, “A/D Precision Requirements for an Ultra-Wideband Radio Receiver”, Proceedings SIPS '02, San Diego, Calif., pp. 270-275, October 2002).
Referring to FIGS. 1 and 4, an example of this impulse radio is described below. FIG. 1 illustrates a monocycle for the impulse radio. A monocycle (unit cycle) 10 is approximated by Equation 1.
                              S          ⁡                      (            t            )                          =                              τ            t                    ⁢                      exp            ⁡                          (                              -                                                      (                                          τ                      /                      t                                        )                                    2                                            )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In this equation, “τ” indicates a time constant for determining the width of the monocycle.
In the impulse radio, communication or instrumentation is performed using a time-series signal in which a monocycle 10 where τ is typically in the range of 0.1 nanosecond to 1 nanosecond is repeated at intervals (e.g. in the range of 10 nanosecond to 1000 nanosecond) longer than τ.
FIG. 2 illustrates an example of a template for the impulse radio. A template 20 is equivalent to the difference between two monocycles being shifted from each other for (20.5)τ. The cross-correlation between this template 20 and the monocycle 10 is illustrated as a graph of a correlation 30 in FIG. 3.
Thus, when the monocycle 10 is received and the correlation between this monocycle 10 and the template 20 is figured out, a correlation value is: 0, provided that the timing of the monocycle 10 is in agreement with the timing of the template 20; positively maximum, provided that the monocycle is ahead of the template for τ/20.5; or negatively maximum, provided that the monocycle is behind the template for τ/20.5. On the basis of this principle, communication or instrumentation using an impulse train having been subjected to pulse position modulation is performed.
Referring to FIG. 4, a method for transmitting binary data by means of impulse radio will be described. In this example, an average interval of transmitting a monocycle is T/N. For spread spectrum and channel multiplexing, the instant of transmitting the monocycle is varied using a pseudo-random code sequence. More specifically, in accordance with the value of the pseudo-random code which is either +1 or −1, it is determined whether the instant of the transmission is retarded for Δt or advanced for Δt, from an instant determined by the average transmission interval.
To transmit sets of binary data +1 and −1 at intervals T, an impulse train 41 indicated by SL(t) is used for the set of data +1. The impulse train 41 repeatedly transmits the monocycle for N times, and instants of the transmission are τ/20.5 later than the above-mentioned retarded or advanced instants.
When the set of binary data −1 is transmitted, an impulse train 42 indicated by SE(t) is used. The impulse train 42 repeatedly transmits the monocycle for N times, and the instants of the transmission are τ/20.5 earlier than the above-mentioned retarded or advanced instants.
To demodulate the data in response to the receipt of the impulse train 41 or 42, a template train 43 indicated by TP(t) is used. This template train 43 is a signal in which the template 20 is repeated for N times, and the templates 20 are generated at the above-mentioned retarded or advanced instants.
A receiver which has received the impulse train 41 or 42 figures out the correlation between the impulse train 41 or 42 and the template train 43. Here, it is assumed that, using an appropriate means (e.g. delay lock loop) for typical communications, the received impulse train 41 or 42 is in sync with the template train 43.
When the received signal is the impulse train 41, a correlation value between each of the monocycles 10 and each of the templates 20 is a negative maximum value so that it is possible to obtain a negative value as a correlation value which is figured out by adding the negative maximum value for N times. In contrast, when the received signal is the impulse train 42, a correlation value between each of the monocycles 10 and each of the templates 20 is a positive maximum value so that it is possible to obtain a positive value as a correlation value figured out by adding the positive maximum value for N times.
For instance, U.S. Pat. No. 5,363,108 to Fullerton, titled “TIME DOMAIN RADIO TRANSMISSION SYSTEM”, discloses a technique for communication and a radar device utilizing impulse radio, as illustrated in a block diagram in FIG. 10.
According to the block diagram in FIG. 10 illustrating a transmitter receiver of the radar device, a received signal is: in a mixer 230, multiplied by a pattern generated by a template generator 232; integrated by an analog integrator 250; and amplified by an amplifier 252. Then the received signal is sampled by a sample and hold 254, quantized in an A/D converter 256, and finally integrated by a digital integrator 262 so that a correlation value is produced.
However, according to this conventional art, since a quantization error arises in the signal each time the A/D converter 256 carries out the quantization, a SN (Signal to Noise) ratio of the signal deteriorates. To improve the SN ratio, it is necessary to increase the quantifying bit number of the A/D converter 256. However, this significantly increases the power consumption.